In recent years, from the viewpoint of energy problems and environmental problems, an artificial photosynthesis technology is under development which electrochemically reduces carbon dioxide using renewable energy of sunlight artificially by modeling photosynthesis of plants to generate a stockable chemical energy source.
An electrochemical reaction device realizing the artificial photosynthesis technology includes, for example, a photoelectric conversion layer using a semiconductor, an oxidation reaction electrode that oxidizes water (H2O) to produce oxygen (O2), and a reduction reaction electrode that reduces carbon dioxide (CO2) to produce a carbon compound. In such a photoelectrochemical reaction device, the oxidation reaction electrode and the reduction reaction electrode which are electrically connected to the photoelectric conversion layer are immersed in water in which CO2 is dissolved, whereby reduction reaction of CO2 is performed.
The oxidation reaction electrode has, for example, a structure in which an oxidation catalyst oxidizing H2O is provided on the surface of a photocatalyst, and obtains potential by light energy. The reduction reaction electrode has, for example, a structure in which a reduction catalyst reducing CO2 is provided on the surface of a photocatalyst, and is electrically connected to the oxidation reaction electrode. The reduction reaction electrode obtains electrons from the oxidation reaction electrode and thereby reduces CO2 to produce carbon compounds such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), methane (CH4), ethanol (C2H5OH), ethane (C2H6), ethylene glycol (C2H6O2) and the like. In a gas reduction product such as carbon monoxide, methane or the like among them, unreacted CO2 is mixed.
The unreacted CO2, when contained in the gas reduction product, affects the purity of a reduction product or the productivity in a synthesis process using them, and therefore it is necessary to remove CO2 in the reduction product. A conceivable method of removing CO2 from the reduction product is a conventional CO2 removing method such as a chemical adsorption method, membrane separation or the like, but the method increases the number of processes and requires large energy for separation, resulting in difficulty in increasing the energy efficiency of the whole system.